1. Field of the Invention
The present invention relates generally to optical networks and fiber optic communications.
2. Related Art
Two types of self-healing data transport network topologies are commonly referred to as "ring" and "mesh." In a ring topology each ring node is connected to exactly two other ring nodes. For self-healing, a ring topology network loops back traffic on a spare path to bypass a break in the ring. The ring topology is attractive for its simplicity and switching speed (around 50 ms).
Traffic is maintained on the spare path until the failure is physically repaired. Under most circumstances, only a few hours are needed to find and repair a link. The time to repair a link stretches to several days, however, when the fiber cut location is inaccessible due to natural disaster, train derailment, plane crashes, etc. During the time that repair is ongoing, the ring is vulnerable to any subsequent failure along the ring. This results because the ring's only spare path has been dedicated to restoration of the first failed link.
In contrast to the ring, a properly equipped mesh network does not exhibit this degree of susceptibility to multiple failures. Mesh structures contain nodes that are connected to more than two adjacent nodes. Thus, traffic between two nodes may traverse one or a plurality of possible routes in the network. Mesh architectures require more sophisticated actions to be taken in response to a failure. If there is a sudden failure of several links, switches in neighboring nodes can perform a coordinated switching to divert the traffic around the failure. For this purpose, most of the spans in a mesh network are equipped with extra spare links that can be called upon in response to a failure. Even though a mesh network offers more flexibility in circumventing failures, the mesh restoration is more complex and more time consuming. Typical mesh networks require on the order of a few seconds to a few minutes to restore a link failure.
Thus, in many installations, a ring network is preferred over a mesh network for providing fast recovery of critical traffic among sites. The simplicity of the ring also makes it easier for network and field engineers to readily predict the risks associated with maintenance activities, such as forced switch over to repair or calibrate a network element.
What is needed, therefore, is a system and method for improving the robustness of a ring network in a multiple failure scenario.